1. Field of the Invention
The present invention relates to fiber optic control systems and, more particularly, to a system that is adapted to transmit signals of a slow digital nature.
2. Description of the Prior Art
There are many control systems where it is important that the transmitting and receiving portions be electrically isolated from each other to protect operators from the danger presented by high voltage equipment. A radio control system could be used in such situations except for the fact that most high voltage equipment usually emits high levels of electromagnetic radiation which would severely interfere with radio communications. Even in the absence of dangerous voltages, both radio and hard-wired control systems are subject to interference by electromagnetic radiation.
The use of optical fibers in such control systems presents a unique solution to these problems since the optical fibers are made of dielectric material that provide electrical isolation between the transmitter and receiving sections. The optical fiber is also unique in that it is not susceptible to electromagnetic radiation and does not pick up interference that would normally be picked up by wires. Thus, when optical fibers are used in sensitive control systems, spurious signals will not result from electromagnetic radiation.
In the prior art, a number of techniques have been utilized in an attempt to transmit several control signals over a single optical fiber in order to reduce the cost of the control link. The most common approach used was to use tone or frequency signaling to differentiate between the various command signals. This technique resulted in extremely high cost systems since it had the inherent disadvantage of requiring accurate alignment of transmit and receive tone frequencies and required accurate frequency calibration. Thus, the systems of the prior art were extremely complex and expensive and therefore made them impractical for many types of applications.